In vitro reassortment and adaptation of influenza A viruses circulating in swine

Since the last influenza pandemic in 2009, H1N1pdm has been introduced into the swine population in Europe where, in combination with swine influenza A virus (IAV) lineages, it started to generate a variety of reassortant viruses of unknown zoonotic risk for humans. To study these reassortment events, we isolated a wild swine lung cell clone (C22) susceptible to IAV infection. We established conditions for co-infection and passaging of H1N1pdm and swine avian-like H1N1. After 7 passages, we plaque-purified C22-adapted strains, characterized their genome composition by next-generation sequencing and analysed replication abilities in swine and human lung cell lines as well as in human lung tissue ex vivo.

Among C22-adapted viruses isolated from co-infection, we revealed reassortants carrying PB1/PA/NA or only PB1/PA from H1N1pdm. We also detected exclusively swine H1N1-derived strains. All isolates carried distinct mutations. As expected, adapted viruses reached higher titers compared to both parental strains in swine lung cells. Furthermore, all C22-adapted viruses were able to replicate in human lung A549 cells without any prior adaptation to the human host. Strikingly, all reassortants were able to infect and efficiently replicate in human lung tissue ex vivo, indicating that these viruses might pose a zoonotic risk.

To summarize, we successfully established an in vitro swine-like model to study reassortment and adaptation of IAVs currently circulating in swine. Our results indicate that our model might be a useful tool to prospectively evaluate the compatibility of different IAV strains to generate reassortants, which might represent a threat to the human population.